All words can be classified as either content or function words, although it is not always easy to make the distinction. With only around 150 function words, 99.9% of words in the English language are content words. Although small in numbers, function words are used at a disproportionately higher rate and make up about 50% of any English text. This is due to the conventional patterns of words usage which bind function words to content words almost every time they are used, creating an interdependence between the two word groups.[1]

Content words are usually open class words, meaning new ones are easily added to the language.[2] In relation to English phonology, content words adhere to the minimal word constraint of being no shorter than 2 morae long (i.e., a minimum length of 2 light syllables or one heavy syllable), while function words do not.[3] Content words always have at least one stressed syllable, whereas function words are often completely unstressed.